Terminal deoxynucleotidyl transferase (TdT) has been shown to be an effective biochemical marker for lymphoblastic cells in normal thymus acute lymphoblastic leukemia (ALL) and lymphoblastic leukemias. The enzyme TdT can be assayed for as an enzyme activity or by an indirect fluorescent antibody procedure. This current project attempted to compare the sensitivity of the two methods of determining the TdT enzyme. In addition, peripheral blood smears, bone marrow aspirate smears and cytocentrifuge slide preparations from a wide variety of diseases as well as normal donors were surveyed for TdT to determine its level in various diseases. Attempts at developing a PAP immunohistochemical assay for localizing TdT in tissue section were successful.